110. Balanced Binary Tree - Easy
Given a binary tree, determine if it is height-balanced.
For this problem, a height-balanced binary tree is defined as:
a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
Example 1:
Given the following tree [3,9,20,null,null,15,7]
:
3 / \ 9 20 / \ 15 7
Return true.
Example 2:
Given the following tree [1,2,2,3,3,null,null,4,4]
:
1 / \ 2 2 / \ 3 3 / \ 4 4
Return false.
先求出子树的高度,如果左右子树的高度差超过1,返回false;如果不超过1,向下递归
time: O(nlogn) -- 求高度O(logn), 一共n个节点, space: O(logn)
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public boolean isBalanced(TreeNode root) { if(root == null) { return true; } if(Math.abs(depth(root.left) - depth(root.right)) > 1) { return false; } return isBalanced(root.left) && isBalanced(root.right); } public int depth(TreeNode root) { if(root == null) { return 0; } return 1 + Math.max(depth(root.left), depth(root.right)); } }
二刷:optimal
class Solution { public boolean isBalanced(TreeNode root) { if(root == null) { return true; } return getHeight(root) != -1; } public int getHeight(TreeNode root) { if(root == null) { return 0; } int left = getHeight(root.left); int right = getHeight(root.right); if(left == -1 || right == -1 || Math.abs(left - right) > 1) { return -1; } return 1 + Math.max(left, right); } }